Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-scaled graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. In a commonly assigned patent, one of the present inventors (Jang) and our colleague (Huang) have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. In most of the prior art methods for making separated graphene platelets, the process begins with intercalating lamellar graphite flake particles with an expandable intercalation agent (also known as an intercalant or intercalate) to form a graphite intercalation compound (GIC), typically using a chemical oxidation [e.g., Refs. 1-5, listed below] or an electrochemical (or electrolytic) method [e.g., Refs. 6,7,17-20]. The GIC is characterized as having intercalate species, such as sulfuric acid and nitric acid, residing in interlayer spaces, also referred to as interstitial galleries or interstices. In traditional GICs, the intercalant species may form a complete or partial layer in an interlayer space or gallery. If there always exists one graphene layer between two intercalant layers, the resulting graphite is referred to as a Stage-1 GIC. If n graphene layers exist between two intercalant layers, we have a Stage-n GIC.) This intercalation step is followed by rapidly exposing the GIC to a high temperature, typically between 800 and 1,100° C., to exfoliate the graphite flakes, forming vermicular graphite structures known as graphite worms. Exfoliation is believed to be caused by the interlayer volatile gases, created by the thermal decomposition or phase transition of the intercalate, which induce high gas pressures inside the interstices that push apart neighboring graphene layers or basal planes. In some methods, the exfoliated graphite (worms) is then subjected to air milling, air jet milling, ball milling, or ultrasonication for further flake separation and size reduction. Conventional intercalation and exfoliation methods and recent attempts to produce exfoliated products or separated platelets are discussed in the following representative references:    1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).    2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).    3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).    4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).    5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).    6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).    7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).    8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).    9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).    10. L. M. Viculis and J. J. Mack, et al., “Intercalation and Exfoliation Routes to Graphite Nanoplatelet,” J. Mater. Chem., 15 (2005) pp. 974-978.    11. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” US Pat. Pending, Ser. No. 11/509,424 (Aug. 25, 2006).    12. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Mass Production of Nano-scaled Platelets and Products,” US Pat. Pending, Ser. No. 11/526,489 (Sep. 26, 2006).    13. Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Method of Producing Nano-scaled Graphene and Inorganic Platelets and Their Nanocomposites,” US Pat. Pending, Ser. No. 11/709,274 (Feb. 22, 2007).    14. Aruna Zhamu, JinJun Shi, Jiusheng Guo, and Bor Z. Jang, “Low-Temperature Method of Producing Nano-scaled Graphene Platelets and Their Nanocomposites,” US Pat. Pending, Ser. No. 11/787,442 (Apr. 17, 2007).    15. Aruna Zhamu, Jinjun Shi, Jiusheng Guo and Bor Z. Jang, “Method of Producing Exfoliated Graphite, Flexible Graphite, and Nano-Scaled Graphene Plates,” US Pat. Pending, Ser. No. 11/800,728 (May 8, 2007).    16. Aruna Zhamu, Joan Jang, Jinjun Shi, and Bor Z. Jang, “Method of Producing Ultra-thin, Nano-Scaled Graphene Plates,” US Pat. application Submitted on Jul. 21, 2007.    17. N. Watanabe, et al., “Method of Producing a Graphite Intercalation Compound,” U.S. Pat. No. 4,350,576 (Sep. 21, 1982).    18. R. A. Greinke, “Expandable Graphite and Method,” U.S. Pat. No. 6,406,612 (Jun. 18, 2002).    19. R. A. Greinke and R. A. Reynolds, “Expandable Graphite and Method,” U.S. Pat. No. 6,416,815 (Jun. 18, 2002).    20. R. A. Greinke, “Intercalated Graphite Flakes Exhibiting Improved Expansion Characteristics and Process Therefor,” U.S. Pat. No. 6,669,919 (Dec. 30, 2003).
However, these previously invented methods [Refs. 1-10, 17-20] have several serious drawbacks:                (a) As indicated earlier, in conventional methods, graphite flakes are intercalated by dispersing the flakes in a solution containing a mixture of nitric and sulfuric acid. The intercalation solution may contain other acidic compounds such as potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, or mixtures of a strong organic acid, e.g., trifluoroacetic acid. After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. The resulting waste water has to be properly treated (e.g., neutralized) prior to discharge into the sewage system. Furthermore, the quantity of intercalation solution retained on the flakes after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. During the high-temperature exfoliation, the residual intercalate species retained by the flakes decompose to produce various species of sulfuric and nitrous compounds (e.g., NOx and SOx), which are undesirable. The effluents require expensive remediation procedures in order not to have an adverse environmental impact.        (b) Typically, exfoliation of the intercalated graphite occurred at a temperature in the range of 800° C. to 1,100° C. At such a high temperature, graphite could undergo severe oxidation, resulting in the formation of graphite oxide, which has much lower electrical and thermal conductivities compared with un-oxidized graphite. In recent studies, we have surprisingly observed that the differences in electrical conductivity between oxidized and non-oxidized graphite could be as high as several orders of magnitude.        (c) The approach proposed by Mack, et al. [e.g., Refs. 9 and 10] is a low temperature process. However, Mack's process involves intercalating graphite with potassium melt, which must be carefully conducted in a vacuum or an extremely dry glove box environment since pure alkali metals, such as potassium and sodium, are extremely sensitive to moisture and pose an explosion danger. This process is not amenable to mass production of nano-scaled platelets.        (d) Most of the prior art intercalation/exfoliation approaches were developed for the purpose of producing graphite worms that are re-compressed to form flexible graphite sheet products. This purpose is perceived to require maximizing the exfoliation volume of a graphite sample. Non-judicious practice of maximizing the expansion volume often occurred at the expense of reduced uniformity in exfoliation, i.e., certain portion of a graphite particle being exfoliated to a great extent, but other portions remaining intact. Graphite worms of this nature are not suitable for the production of separated, nano-scaled graphene platelets.        (e) Although prior art intercalation-exfoliation methods might be capable of sporadically producing a small amount of ultra-thin graphene platelets (e.g., 1-5 layers), most of the platelets produced are much thicker than 10 nm. Many of the NGP applications require the NGPs to be as thin as possible; e.g., as a supercapacitor electrode material. Hence, it is desirable to have a method that is capable of consistently producing ultra-thin NGPs.        
Hence, an urgent need exists to have environmentally benign intercalates that do not lead to the effluent of undesirable chemical species into the drainage (e.g., sulfuric acid) or into the air (e.g., SO2 and NO2). It is further desirable to have a graphite intercalation compound that does not require a high exfoliation temperature. It is also desirable to have a GIC that can be more uniformly exfoliated for the production of nano-scaled graphene platelets that are more uniform in sizes. It is highly desirable to have a method of expanding a laminar (layered) compound or element, such as graphite and graphite oxide (partially oxidized graphite), to produce ultra-thin graphite and graphite oxide flakes or platelets, with an average thickness smaller than 2 nm or thinner than 5 layers.
In order to meet these goals, we investigated potentially viable intercalates that contain no undesirable elements, such as N, S, P, As, Se, transition metal, or halogen element. In particular, we focused our studies on chemical species that contain only H, C, and O atoms, which are expected to produce no contaminants. We have found that carboxylic acids, such as formic, acetic, propionic, butyric, pentanoic, and hexanoic acids and their anhydrides, are particularly suitable for meeting our objectives.
It may be noted that Kang, et al [Ref. 7] used an electrochemical method to intercalate natural flake graphite with formic acid for the purpose of producing flexible graphite products. However, there was no indication that the formic acid-intercalated graphite could lead to well-separated, nano-scaled graphene platelets (NGPs), let alone NGPs of uniform sizes or ultra-thinness (e.g., thinner than 2 nm). Furthermore, there was no indication, implicit or explicit, that any other member of the carboxylic acid series or any member of their anhydrides and their derivatives could be successfully intercalated into interstices of graphite. There was also no indication that any of the carboxylic acid can be intercalated into other layered graphite structures (e.g., graphite fibers, carbon nano-fibers, synthetic graphite, or highly pyrolytic graphite flakes) than natural flake graphite.
It may be further note that, although Greinke, et al [Ref. 18-20] used a carboxylic acid as an “expansion aid” in the formation of expandable graphite (i.e., GICs) for the purpose of producing flexible graphite products, the intercalate in these GICs was sulfuric acid. The method comprises “contacting graphite flake with an organic expansion aid” [Claim 1 of Ref. 18]. It is speculated that the organic expansion aid (e.g., carboxylic acid), under the experimental conditions of Greinke, et al., resided on the exterior surface of graphite particles or between graphite particles. There was no indication that carboxylic acid, when used alone or as a portion of an intercalation solution in [Refs. 18-20], could penetrate and stay in graphite interstices to form a stable GIC. The organic expansion aid was used to increase the macroscopic expansion volume of a graphite sample, not for improving uniform expansion of individual flakes in a graphite particle, nor for enhancing the separation of exfoliated flakes. There was no attempt to apply this approach to exfoliation of other graphite structures than natural flake graphite. There was no attempt on submitting the exfoliated graphite to a mechanical shearing treatment for the purpose of producing NGPs.
By contrast, after intensive studies, we have observed that a significant amount of a member of the carboxylic acid family, when assisted by an oxidizing agent such as hydrogen peroxide, can be intercalated into graphite to form a stable graphite intercalation compound (GIC). The GIC, when exposed to a temperature in the range of 300-800° C. (preferably in the range of 400-600° C.), was exfoliated in a relatively uniform manner. The resulting exfoliated flakes can be readily separated, via a mechanical shearing treatment, into individual nano-scaled graphene platelets (NGPs) that are relatively uniform in thickness. This was achieved without using an undesirable acid like sulfuric acid or undesirable oxidizing agent like nitric acid or potassium permanganate.